Parasitic capacitance of stripboard

[ianb1469]

We are always told that stripboard has a relatively "high parasitic capacitance" between tracks, but how high?

I searched the internet expecting to find many references to "typical capacitance ... per mm of adjacent track", but not even the manufacturers seem to provide this information.

If I knew a little about the capacitances on a protoboard, then I'd be able to know about whether I need to consider it for the sort of frequencies in the various parts of a circuit.. when to spend extra effort laying out the tracks and getting impedances worked out carefully and when to just get on with it.

Thanks for any advice.
Ian

 

[Resqueline]

Good points, we can't leave that uninvestigated!
Measured on a 307mm length of VERO 01 0042 WK 29 97:
7.5pF between every other track.
13pF between adjacent tracks.
22pF between a track and two grounded tracks, one on either side.

 

[ianb1469]

Hi

Many thanks for making those measurements! (Out of interest: how? Certainly my old/faithful multimeter won't measure that low).

Anyway, based on your measurements (the 22pF one) we might estimate the capacitance to be about 0.2pF per 0.1" hole.

It's shockingly high; you don't have to get to particularly high frequencies before the signal vanishes.

Many thanks for the measurements, you've saved me from prototyping an idea I had on stripboard; it would simply not work

Regards
Ian

 

[Resqueline]

I have a Philips multi-frequency RCL/ESR meter at work. I took those measurements at 1kHz. I meant to do it all week but was kept so busy I forgot until today.
Glad to be of help. If you want to know the track inductance + resistance (up to 100kHz, including skin effect) i can measure that too.
What frequency would you estimate is the highest practical for these Vero boards btw.?

 

[ianb1469]

>What frequency would you estimate is the highest practical for these Vero boards btw.?

Well, of course it depends...

For my particular application, I am stuck with basically an RC filter arrangement and needing to feed signals at about 20kR impedance. If I can apply anything I once was supposed to have learnt, then at even 4 holes, the corner frequency is 10MHz. At a more realistic 20*0.1" length of track, we're down to 2MHz.

I suppose that if you can get the load resistance down to 1k then it would be practical at 40Mhz.

Does that sound about right? It's many years since I went to university!

 

[Resqueline]

Yes of course, silly question, with a fixed capacitance the impedance is the all-determining factor for the corner frequency. I checked your numbers and they do add up.
So if doing 75 ohm stuff then 20 holes could possibly be good up to around 500MHz.

 

[ianb1469]

500MHz - I suspect that's rather optimistic

 

[Resqueline]

Seeing how UHF circuits are made up I guess you are right..